1. Field of the Invention
The present invention relates to a tape end detector for detecting a tape end by detecting difference between the optical transmissivity of a transparent tape leader and the optical transmissivity of a magnetic substance applied part using phototransistors, and a video cassette reproducing apparatus provided with the above tape end detector.
2. Description of the Related Art
In a video cassette deck, a phototransistor is used to detect a transparent leader at a tape end, however, the phototransistor receives light weak in intensity in relationship with an optical path. As a result, the phototransistor does not become a saturated state even if it receives light. Therefore, the phototransistor for detecting a tape end cannot be used for a switch for transferring its state between two states of an ON state and an OFF state. Therefore, output from the phototransistor is required to be dealt as an analog signal and two types of analog input composed of analog input for a phototransistor for detecting a tape end on the side of a take-up reel and analog input for a phototransistor for detecting a tape end on the side of a supply reel are required. However, as the two types of analog input are required in the case of the above configuration, the configuration of a microcomputer becomes complicated.
Technique to solve the above problem is proposed in Japanese Utility Model Registered Publication No. 3055795. That is, the voltage of a positive power source is divided by a voltage dividing circuit composed of a first resistor and a second resistor. The first phototransistor for detecting a tape end on the side of the take-up reel is connected to the first resistor in parallel. The second phototransistor for detecting a tape end on the side of the supply reel is connected between a divided voltage output point of the voltage dividing circuit and a direction output terminal of a microcomputer. Therefore, when the direction output terminal of the microcomputer is at a low level, three types of states of a state in which only the first phototransistor is turned on, a state in which the first and second phototransistors are both turned off and a state in which only the second phototransistor is turned on can be discriminated only by detecting voltage at the divided voltage output point.
However, if the above configuration is used, the following problems occur. That is, even if a phototransistor receives beams equal in intensity, a value of resistance between the collector and the emitter of the phototransistor greatly varies due to dispersion between the characteristics of the phototransistors. As for the optical transmissivity of a transparent leader of a video tape, 50% or more meets a standard value and as to the optical transmissivity of a magnetic substance applied part, 1.5% or less meets a standard value. Therefore, if the dispersion of optical transmissivity and the dispersion between the characteristics of phototransistors are synergistic, difference between a value of resistance between the collector and the emitter when light transmitted through the tape leader is received (at on time) and a value of resistance between the collector and the emitter when light transmitted through the magnetic substance applied part is received (at off time) is double or less as ratio.
Therefore, a range of the dispersion of voltage at the divided voltage output point in case the phototransistor on the side of the take-up reel and the phototransistor on the side of the supply reel are both turned off overlaps with a range of the dispersion of voltage at the divided voltage output terminal when one phototransistor is turned on. Therefore, an error may occur in discriminating two types of states of a case that two phototransistors are both turned off and a case that one phototransistor is turned on. To prevent such an error in discrimination, it is required that the above two types of ranges of dispersion are not overlapped and it is enabled by selecting and using phototransistors equal in characteristics. However, if a phototransistor is selected and used, the price of the phototransistor is increased.
The present invention was made to solve the above problems and an object of the present invention is to provide a tape end detector wherein a tape end can be detected without increasing the number of output terminals of a microcomputer and without an error even when elements large in dispersion between characteristics are used.
To achieve the above object, according to the present invention, there is provided a tape end detector for detecting a tape end based upon difference between an optical transmissivity of a tape leader and an optical transmissivity of a magnetic substance applied part, comprising: a first resistor one terminal of which is connected to a positive power source; a second resistor one terminal of which is connected to the other terminal of the first resistor and the other terminal of which is grounded; a microcomputer to the analog input terminal of which the other terminal of the first resistor is connected; a first phototransistor the collector of which is connected to the output terminal of the microcomputer and the emitter of which is connected to the other terminal of the first resistor for detecting one tape end on the side of a take-up reel or on the side of a supply reel; and a second phototransistor the collector of which is connected to one terminal of the second resistor and the emitter of which is connected to the output terminal of the microcomputer for detecting the other tape end on the side of the take-up reel or on the side of the supply reel, wherein the microcomputer determines whether the first phototransistor is turned on or off based upon voltage applied to the analog input terminal when the output terminal is at a high level, and determines whether the second phototransistor is turned on or off based upon voltage applied to the analog input terminal when the output terminal is at a low level.
That is, when the output terminal is at a high level, voltage applied to the analog input terminal depends upon only a value of resistance between the collector and the emitter of the first phototransistor and is not influenced by the second phototransistor. When the output terminal is at a low level, voltage applied to the analog input terminal depends upon only a value of resistance between the collector and the emitter of the second phototransistor and is not influenced by the first phototransistor. That is, a state in which the first phototransistor receives light and a state in which the second phototransistor receives light are individually determined by one output terminal the level of which can be changed to a high level and a low level and one analog input terminal without being influenced by the other phototransistor.